The invention is based on a priority application EP 05291727.5 which is hereby incorporated by reference.
The invention relates to a method for monitoring an optical transmission line of an optical data network, wherein light is sent into the optical transmission line at a line input, and backscattered light of said light is detected at said line input and analysed. The invention further relates to a measuring device and an optical transmitter.
Such a method has been disclosed, for example, in a lecture of W. Bludau, “Messverfahren zur Charakterisierung von Lichtwellenleitern und-komponenten”, at the Symposium “Nachrichtenubertragung mit Lichtwellenleitern”, Technische Akademie Esslingen, Oct. 18-20, 2000, Ostfildern-Nellingen (Germany).
Optical networks are used to transfer digital data. For this purpose, an optical network requires fully functional optical transmission lines. Therefore, the optical transmission lines need to be checked, in particular upon installation and in case of problems with the data transfer on a particular transmission line.
A method to check the state of an optical transmission line is optical time domain reflectometry (OTDR), as disclosed in the lecture of W. Bludau, see above. Short test pulses of light are sent into an optical fiber in order to activate Rayleigh back scattering within the optical fiber. The backscattered light is detected. A plot over time of the backscattered light is, when the optical fiber is fully functional, an exponentially declining curve. In case of defects within the optical fiber, abrupt changes are registered within the curve. The time delay between the illumination at the fiber input and the registry of back-scattered light indicates the position within the fiber where a problematic scattering event has taken place. In order to check the state of a fiber with this known method, it is necessary to interrupt the data transfer service, in particular for sending the short test pulses, and to reconfigure fiber connections manually; this makes this method cumbersome and expensive.
In a modified OTDR method, a sine wave modulation is done on top of a continuous data modulation (=super modulation), and the complex frequency characteristics of the backscattered channel are measured, followed by an inverse Fourier transformation. Thus the required impulse response is obtained. This modified method uses an operational data channel, but it reduces the extinction ratio of the data modulation. Further, it requires a linear operation of the modulator and its driver. Linear operating drivers and modulators are rather expensive compared to usual optical data transmitters which use nonlinear, on/off modulation techniques.